In packet transmission systems and/or switching systems, it is important to delimit packets, i.e., identify the beginning and end of the packets. Packets were delimited in prior arrangements by employing flags, i.e., a specific code word, placed at the beginning and end of the packet and by employing bit stuffing so that data being transmitted did not emulate the flags.
Such prior delimiting arrangements require bit-by-bit processing. Consequently, they are difficult, if at all possible, to implement for high bit rate packet transmission systems, e.g., bit rates of 45 Mbits/sec or higher. Indeed, any possible implementation of such a prior delimiting arrangement would be expensive and, therefore, undesirable from an economic standpoint.
More recently, packets have been delimited by employing a unique packet transport word, i.e., header, including one or more prescribed parity checks in conjunction with a packet length, i.e., size, indicator, as described in my U.S. patent application Ser. No. 371,236 filed concurrently herewith. In such a packet transport word arrangement, it is possible that the user information may emulate a possible packet transport word. The probability of user information emulating a public network packet transport word is made extremely low by employing a relatively "large" number of check bits in the parity checks.
However, if a particular transport user employs transport words in a private or other network which are similar to those used in the public network, it is possible that such private network transport words will be mistaken for those used in the public network. Such occurrences are extremely undesirable and must be avoided. Otherwise, errors in transmission and/or switching will occur. That is to say, packets would be lost or, worse, miss-delivered.